1. Field of the Invention
The present invention relates to a sheet post-processing apparatus for applying a specified post-processing to sheets fed from an upstream apparatus such as an image forming apparatus and discharging the post-processed sheets to a discharge tray.
2. Description of the Related Art
There has been adopted a sheet post-processing apparatus for automatically applying a specified post-processing such as punching and stapling to sheets discharged from an upstream apparatus such as an image forming apparatus. In the case of using such a sheet post-processing apparatus together with the upstream apparatus, the sheet post-processing apparatus is normally united with the upstream apparatus into an integral unit while being closely attached to the upstream apparatus. Accordingly, the sheets conveyed from the upstream apparatus are generally discharged onto a discharge tray provided in the sheet post-processing apparatus after passing the inside of the sheet post-processing apparatus regardless of whether or not they are to be post-processed. Such a sheet post-processing apparatus is known from Japanese Unexamined Patent Publication Nos. H07-53115, H09-263355 and 2002-68553.
The sheet post-processing apparatus disclosed in Japanese Unexamined Patent Publication No. H07-53115 is constructed such that an angle of inclination of the discharge tray provided in the sheet post-processing apparatus is made variable depending on whether or not the sheets conveyed from the upstream apparatus are to be post-processed, thereby changing an installation angle of the discharge tray depending on whether the sheets are discharged without having any post-processing applied thereto or they are discharge after having no post-processing applied thereto. Such an arrangement is made because a sheet discharge opening in the case of applying the post-processing to the sheets and the one in the case of applying no post-processing are provided at different positions, and the angle of inclination of the discharge tray is made variable so as to conform to both sheet discharge openings.
The sheet post-processing apparatus disclosed in Japanese Unexamined Patent Publication No. H09-263355 is constructed such that, upon an abnormality in a stacked state of sheets discharged from the sheet post-processing toward the discharge tray, such an abnormality is detected and the discharge tray is moved downward or upward to solve the abnormality in the stacked state by this movement.
The sheet post-processing apparatus disclosed in Japanese Unexamined Patent Publication No. 2002-68553 is constructed such that whether or not an end portion of a sheet discharged from the sheet post-processing apparatus is turned up to have U-shaped curl is detected, and the height position of the discharge tray is lowered to avoid the interference with a succeeding sheet.
However, since the angle of the discharge tray is merely changed depending on whether or not any post-processing is to be applied to the sheets in the sheet post-processing apparatus disclosed in Japanese Unexamined Patent Publication No. H07-53115, there is a problem that the sheet discharge cannot be properly performed due to the interference of the curl with the succeeding sheet if the sheet is curled.
Further, since the discharge tray is moved upward or downward after detecting the abnormal discharge of the sheets in the sheet post-processing apparatus disclosed in Japanese Unexamined Patent Publication No. H09-263355, only a measure against the already occurred abnormality is given upon an abnormal sheet discharge, for example, resulting from the curled sheet. Thus, there is a problem of being unable to prevent the sheet discharge abnormality.
In the sheet post-processing apparatus disclosed in Japanese Unexamined Patent Publication No. 2002-68553, the discharge tray is lowered to avoid the interference with the succeeding sheet if the sheet is curled upward to become U-shaped. Whether or not the sheet is curled is detected by a curl sensor. However, it is difficult to detect a slight curl formed at an end portion of the sheet by means of the curl sensor. Accordingly, there is a problem that the curled sheet may be handled as the one having no curl due to an erroneous detection of the curl sensor despite the actual presence of the curl.